Step into an old oak wood on the rainy western side of Britain and the trees look upholstered. Trunks, boulders, the banks of every stream and the low branches overhead all wear a deep, springy pelt of green. A good part of that pelt, especially the tresses trailing down the lower trunk, is mouse-tail moss (Isothecium myosuroides), one of the plants that gives Atlantic woodland its furred, dripping character.
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How to know it
The name is a fair description, provided you meet the moss in dry weather. Long creeping stems run over bark or stone and throw up crowds of slender branches, each tapering to a fine incurved tip, so a dry patch bristles with little pointed tails that mostly lean the same way. Wet it, and the branches plump out and soften and the whole colony deepens to a wet green; let it dry, and it pales while the tails tighten and hook over, which is the moment the mouse-tail likeness reads best. The leaves themselves are small ovals pulled out to a point, each carrying a single nerve that runs perhaps two-thirds of the way up the blade, though you want a hand lens for that, and in practice the branch shape names the plant well before the leaves do.
Where it lives
Shade and steady damp are what it asks. You find it on the lower trunks and buttress roots of trees, on sandstone and other hard acid rock, on shaded walls and down the sides of wooded ravines, wherever the air seldom dries out for long. On a tree it tends to clothe the first metre or two of the bole and the swelling roots, thinning higher up as light and wind take over. It grows across most of Britain and Ireland, but it truly comes into its own in the humid west and the uplands, where whole boulder fields and stream gorges can be sheeted in it end to end.
Its part in the wet woods
In the coastal rainforests I write about under Atlantic temperate rainforest, mouse-tail moss does much of the unshowy work of building the carpet. It rarely counts as a prize in its own right, yet it is the bedding on which the prizes settle: its dense, water-holding weft keeps trunk and rock humid between showers, and the scarcer liverworts, filmy ferns and specialist mosses take root and perch among its stems. Clear that carpet away, as happens where these woods are felled or grazed too hard, and the fussier species lose the very thing that let them live there. A gentler version of the same structural job goes on wherever moss softens a tree, which I take up under moss on trees.
Telling it apart
Two look-alikes are worth heading off. The first is its own bigger cousin, larger mouse-tail moss (Isothecium alopecuroides), a stouter plant of base-rich rock and old tree bases whose branches end blunt and club-shaped rather than drawn to a fine point; where the two share a boulder the difference in heft usually settles it. The second is any of the trailing feather-mosses that haunt the same bark, common feather-moss (Kindbergia praelonga) among them, which spread in flatter, more evenly branched sprays and never gather into those bunched, one-way tails. Being a pleurocarp, mouse-tail moss fruits from side shoots rather than from the growing tip, a habit I explain once under acrocarpous versus pleurocarpous. Its capsules, narrow and held upright to slightly leaning on a reddish stalk, turn up mainly in winter and are not especially frequent, so most of your naming will rest on the growth habit rather than the fruit.
Growing it, or letting it be
You are unlikely to plant this one on purpose, and it does not care to be asked. It belongs to humid, undisturbed places and sulks in the dry, bright corners most gardens offer; a lifted clump pressed onto a shaded stone or a north wall may cling on, but it seldom flourishes the way it does among the trees. For the look of moss-clad rock closer to home, the tougher and far more forgiving species I gather under moss on rocks will repay you sooner. My own preference is to meet mouse-tail moss where it belongs. Go out after a wet spell, hold a lens to the bark of an old western oak, and watch the tails swell and unhook as the moss drinks. It is a small thing to witness, and it explains at once why these woods feel the way they do.